1. Field of the Disclosure
This disclosure relates to a photo diode for detecting x-rays.
2. Description of the Related Art
The diagnostic x-ray cameras being currently used in a medical field take a photograph using a screen film. The x-ray photograph screen film must be developed in order to display an x-ray photograph image. Recently, digital x-ray detectors have been developed and researched which are configured to use thin film transistors based on semiconductor technologies.
Also, radioactive ray detectors are ordinarily configured to detect radioactive rays penetrating through the human body, so as to obtain image information. To this end, the x-ray detector includes an x-ray detection flat-substrate corresponding to a detection panel which converts radioactive rays with the image information into electrical signals. Similarly, x-ray image apparatuses must include a detection element configured to detect x-rays passing through an object and convert the detected x-rays into electrical signals.
The detection element is formed in a flat panel on which a plurality of unit cells each having a thin film transistor are used as a detection pixel and arranged. As such, the detection apparatus using the TFT substrate sequentially selects gate electrodes of the thin film transistors in one column and reads electrical signals detected by the arranged pixels in one column, in order to obtain an image data including the electrical signals for the pixels. Also, the detection apparatus applies the electric pixel signals included in the image data to a display device such as a monitor or others, thereby providing a digital image.
More specifically, each of the detection pixels arranged on the x-ray detection panel include a photoconductor configured to generate electric charges in proportion to the amount of irradiated x-rays, and a collection electrode configured to collect the electric charges generated in the photoconductor. The detection pixel further includes a capacitor configured to charge the collected electric charges collected by the collection electrode, and a switching element configured to selectively transfer the electric charges charged into the capacitor to a read-out line. The photoconductor is used to convert the x-ray into an electric signal. In detail, the photoconductor generates pairs of electrons and holes corresponding to the x-rays. Such a photoconductor is formed from selenium with a light-to-electric converting property.
The switching element is implemented to include a thin film transistor. The thin film transistor includes a gate electrode connected to a gate line and a source electrode connected to a read-out line. When the electric charges generated by incident x-rays are charged into the capacitor, the thin film transistor outputs a voltage signal charged into the capacitor to the read-out line, so that a photographed image can be reproduced.
FIG. 1 is a cross-sectional view showing the structure of a pixel on an x-ray detection panel of the related art. Referring to FIG. 1, the pixel includes an insulation layer 12 formed on a substrate 10, and a drain electrode 13 of a thin film transistor disposed on the insulation layer 12. The pixel further includes a cathode electrode 20 disposed opposite the drain electrode 13 in the center of a first protective layer 15, a photoconductor layer 25 formed on the cathode electrode 20, and an anode electrode 30 disposed on the photoconductor layer 25 opposite to the cathode electrode 20. The pixel still further includes a second protective layer 32 formed to cover the anode electrode 30, and a power line 40 disposed on the second protective layer 40 and connected to the anode electrode 30.
The cathode electrode 20, photoconductor layer 25, and anode electrode 30 form a photo diode which converts x-rays or natural light irradiated from the external into an electric signal and charges the converted electric signal. The charged electric signal is output to a read-out line (not shown) when a driving signal is applied to the gate electrode of the thin film transistor disposed in a pixel region. Therefore, the charged electric signal can be used to display an image.
Such a photo diode is largely subjected to a leakage current in its electrical performance. This results from the fact that the power line 40 is used to bias the photo diode. If the leakage current greatly increases, the electrical performance of the photo diode seriously deteriorates. Particularly, it is issued a leakage current generated along the edge of the photo diode entirely occupying the pixel region.